Directional drilling, subsurface boring, microtunneling and the like share the objective of producing a hole beneath the surface of the earth in a carefully controlled manner. The hole is usually required for installation of tubular members such as pipes (for natural gas, petroleum fluids, steam, water, other gases and liquids, slurries, or sewer connections), protective conduits (for subsequent installation of electrical wires, cable television lines or fiber optic cable), for direct burial of electric wires and fiber optic cable, and the like. All such activities, particularly those in the top most one hundred feet or so of the subsurface, are described as subsurface boring.
Subsurface boring is increasingly important because it allows rapid placement or replacement of gas and water lines, sewers, electrical service, cable television service and similar utility connections with minimal disturbance of roads, landscaping, buildings, and other surface features. Subsurface boring allows placement of pipe and utility connections where conventional surface installation by trenching is impractical or impossible as, for example, when utility connections must cross rivers, canals, major highways, or rail lines. Significant practical and economic advantages are derived from the ability to provide pipe and utility connections with minimal surface disturbance or to provide subsurface crossing of surface barriers.
Locating and communicating with the boring tool are problems that must be solved to realize all advantages of subsurface boring. In many boring tool systems, a small transmitting device called a beacon is installed immediately behind the drill bit. The electromagnetic field generated by the subsurface beacon is detected at the surface using devices variously called trackers or locators. The beacon's electromagnetic field is often modulated to convey subsurface information from the beacon to the system operator. Beacon signal amplitudes, measured by one or a plurality of surface antennas, can be used to calculate distance between the beacon and the tracker or locator. Depending on beacon modulation details, the subsurface beacon transmits on or around one of several possible frequencies. It is common to allow operation at one of at least two different frequencies, thereby allowing two boring operations to proceed simultaneously in close proximity while still being able to differentiate between the two by using different beacon frequencies.
As operating depth increases, locating and communicating become increasingly difficult because, in the near field, signal amplitude decreases according to the third power of distance. As in any communication system, the signal-to-noise ratio is a primary determinant of success in signal estimation and communication. Beacon power is limited, hence available signal is limited. Accordingly, noise reduction assumes great importance. With limited signal power, the only way to improve the signal-to-noise ratio is to reduce the noise.
The noise figure of the receiving channel in the tracker or locator is determined largely by the preamplifier. Therefore, improvements in the preamplifier relating to signal handling and noise rejection are of great significance to the art.
U.S. Pat. Nos. 5,155,442 and 5,337,002 by Mercer, disclose locator equipment for detecting the magnetic field transmitted by a boring tool of the type that forms a horizontal bore under the surface of the earth. The locator equipment is either hand held, in which case it is maneuvered over the surface to detect the magnetic field transmitted by a small battery-operated transmitter carried by the boring tool, or it is surface mounted at some location determined by the equipment operator. The locator equipment includes a conventional parallel LC resonant circuit to selectively receive the fundamental frequency associated with roll/pitch information transmitted by the boring tool. The capacitor component of the resonant circuit is apparently variable to either change the resonant frequency of the receiver, or to change the resonant frequency to compensate changes due to aging, temperature, calibration, etc. The shortcomings of such an arrangement are noted below and are overcome by the present invention.